Angle switching device fob syn



NOV. 19, 1940. Q Q SHUTT Re. 21,629

ANGLE SWITCHING DEVICE FOR SYNCHRONIZING TWO SYNCHRONOUS MOTORS Original Filed July 16, 1935 2 Sheets-Sheet 1 WITNESSES: 1 INVENTOR d 7.56 Char/e5 C677 aft 6% BY ATTORNEY Reiuiled Nov. 19, 1940 UNITED STATES PATENT OFFICE ANGLE SWITCHING DEVICE FOR SYN- CHRONIZING TWO SYNCHRONOUS M- TOBS Pennsylvania Original No. 2,175,923, dated October 10, 1939, Se-

rial No. 31,614. July 16, 1935. Application for reissue September 28, 1940, Serial No. 358,892

3 Claims.-

My invention relates to means for eflecting a given relative operating position of two or more shafts which are to operate in synchronism.

More particularly, my invention relates to 5 electric control systems whereby the relative rotor positions at a given instant of two or more electric motors operating in synchronism is effected.

It often happens that the load characteristics,

namely, the variations of torque of the load driven 1 by one prime mover has periodic variations.

When several such loads are to be coupled to a prime mover it is desirable that the driven shafts be coupled to the prime mover in such relative angular relation with reference to corresponding planes including the driven axes that the peak torques of all the loads do not occui at the same time.

One object of my invention is to provide for driving two or more shafts in synchronism but in such relative relation that corresponding points on the shafts hold selected positions with reference to corresponding planes including said respective shafts.

The advantages of my invention are particularly noticeable in conjunction with the operation of a plurality of synchronous motors driving loads of varying character, as for instance compressors.

Synchronous motors are, as a rule, started as induction motors and at or near synchronism transferred from induction motor operation to synchronous motor operation. If several synchronous motors are connected to the same source of supply, the desirable starting operation is that all the motors are not synchronized at the same time. Furthermore, if the motors are coupled to loads that vary periodically it is very desirable that the peak loads on the motors do not occur at the same instant.

For instance, if two or more synchronous motors are to operate compressors, it would not be desirable that the crank arms occupy the same relation during synchronous operation. If the motors could be synchronized so that corresponding poles hold different positions at a given instant during synchronous operation, it is apparent that the peak loads for the respective motors would occur at. different times.

One object of my invention is to so synchronize a plurality of synchronous motors that corresponding pole pieces hold selected positions at a given instant during synchronous operation.

Another object of my invention is to control the successive synchronization of a plurality of motors so that corresponding points on the shafts of the motors hold selected angular positions at a given instant with reference to corresponding planes including the axes of the respective motors.

Another object of my invention is to synchro- 5 nize a synchronous motor at such an instant that its shaft will hold a given relation to the shaft of another synchronous motor operating at synchronism.

A still further object of my invention is to con- 10 trol the successive synchronization of synchronous motors by electric discharge means that are made responsive to the relative angular disposition of the shafts at a given instant.

Other objects and advantages of my invention 15 will become more apparent from a study of the following specification when taken in conjunction with the accompanying drawings, in which:

Figure 1 shows a starting control scheme for a synchronous motor, and a portion of the 20 combined control scheme constituting my invention; and

Fig. 2 shows a starting control scheme for a synchronous motor, and the remaining portion of the combined control scheme constituting my 25 invention.

Referring more particularly to Figs. 1 and 2 of the drawings, the reference character l1 designates a synchronous motor which may be connected to the source of supply represented by the conductors I, 2 and 3 by suitable operation of the contactor 1. In the particular arrangement shown in both figures tim limit control means are shown for both motors l1 and I10. P The time limit device in Fig. 1 is represented by an electromagnetic device I! provided with a dashpot 53. The field winding of the motor is arranged to be connected to a pair of direct cur-' rent buses BI and 64 by the main field switch 30 and a high speed field switch 231. The high 40 speed field switch 231 is suitably controlled by the electric discharge device 215.

In Fig. 2, motor I10 may be similarly interconnected with the source of alternating current represented by the conductors I, 2 and 3 by the 45 contactor I01,'a'time limit device 3I2 is shown for delaying the energization of the field winding with direct current from the direct current buses BI and 64 by the main field switch 45.

A high speed field switch 208 may also con- 50 nect the field winding of motor I10 to the source of direct current when the electric discharge device I15 becomes conducting.

As hereinbefore pointed out in the objects, it is one of the desirable features of my system of 65 control to synchronize either one of the two motors shown whm the other is operated in synchronism at such an instant that the shaft of the motor last to synchronize will hold a definite relation with reference to the shaft 01 the motor first to synchronize.

In the drawings, motor "is provided with an impulse generator 23, whereas motor I13 is provided with an impulse generator I23. These impulse generators are so arranged that their voltage impulses occur at the same instant when certain points on the shafts of the respective motors hold diiierent angular positions with reference to the axis of the motor shafts and corresponding points on the stators.

The impulse generators are arranged to vary the grid bias of the electric discharge devices shown and the arrangement of the circuits is such that the particular electric. discharge device that is to be caused to operate will only operate when the voltage impulses act concurrently or additively.

As hereinafter explained more in detail, when one of the motors is caused to synchronize, certain electro-magnetic devices and one of the electric discharge devices is disconnected so that it is not utilized for the synchronization of the motor last to be synchronized, but the scheme of control is such that no particular order of synchronizetion of the motors need be selected. Motor I19 may be started first or motor l1 may be started first. If motor I'll is started first, electric dis-' charge device 215 in Fig. 1 is the device utilized for controlling the synchronization of motor l1. Furthermore, it motor I1 is synchronized first. the electric discharge device I" is utilized to synchronize motor I10.

If motor I1 is operating at synchronous speed and motor I10 has attained its balance speed operating as an induction motor and the time limit device 3I2 has operated, the impulse generators will cause the operation of the electric discharge device I and synchronize motor I13 at such an instant that the peak loads on the two motors will not occur at the same time.

A better understanding of the novel features of my invention can probably be had from a. study of typical operating sequences for the two motors that are to be energized and synchronized in such a manner that corresponding pole pieces are disposed in different relation on the two motors or what is more important that the crank arms on the shafts of the motor he in different operating relation.

Assuming that conductors I, 2 and 3 are suitably energized by an alternator or be energized from conventional power means, and the attendant wishes to set motors l1 and I" in operation in the desired manner. Further, assuming for the first sequence of operation that the attendant wishes to start motor l1 first. He thereupon actuates the push button switch I thereby establishing an energized circuit from conductor 3 through conductor 4, push button switch or starting switch 5, actuating coil 3 of the contactor 1, stop switch 3 and conductors 9 and II to the enersized conductor 2. Energization of the actuating coil 8 causes the operation of the contactor 1 thereby closing the contact members ll whereby a holding circuit is established for the actuating coil 6 through the contact members II and the starting switch 3 may be released.

Further, the operation oi the contactor 1 closes the contact members l3, l4 and I5 thereby connecting the primary winding II to the conductors I, 2 and 3 to thus energize the motor II. In the particular motor shown, the primary constitutes the stator whereas the rotor It carries the direct current field of the motor l1.

My system 01' control is in this application shown in conjunction with a pair of synchronous motors of like design. However, it is apparent that sane of the novel results hereinbefore pointed out and to be pointed out further hereinafter may be obtained even in connection with other than synchronous motors.

The rotor has a shaft l9 to which the load may be coupled but which also may carry one, or as shown a pair of bars 29 and 2| of magnetizable material of an impulse generator 23 adjustably mounted on'the shaft l9 by the adjusting means 24. The impulse generator 23 has a stator member 29 disposed adjacent the bars and 2| and rigidly secured to the stator It by the securing means 28. I: it be assumed that the synchronous motor I1 is driving a compressor, it will be readily apparent that the load is of a very fiuctuating character and comes to a high value during certain portions of the revolution or often a certain point during each revolution of the motor shaft. By properly positioning the members 20 and 2| with reference to the stator 25, impulses may be generated in the coils 21 and 23 mounted on the stator 25, which impulses will have a definite time relation with reference to the position of the motor shaft l9.

As heretofore explained, energization of the primary I 6 starts the synchronous motor l1 as an induction motor with the result that the alternating current induced in the field winding discharges through the discharge circuit including contact members 29 of the field switch 39, discharge resistor 3| and conductor 32. "*Qperation of the contactor 1 also closes the contact members 33 thereby, establishing a circuit from the energized conductor 4 through contact members 33, conductor 34, back contact members 35 of the sequence control relay 36, conductor 31, actuating coil 38 of the sequence control relay 39 and conductor 40 to the energized conductor 2. Operation of the sequence control relay 39 causes the opening f the back contact members 4| which, therefore, places an open circuit in the circuit for the actuating coil 42 of the sequence control relay 33 so that this relay cannot be caused to operate once the attendant has selected motor I! as the motor first to operate.

Operation of the sequence control relay 39 opens the contact members 43 thereby placing an open circuit in one of the possible energizing circuits for the actuating coil 44 of the field contactor 45, with the result that the field contactor 45 cannot be energized by a circuit including the contact members 43. Further, the operation of sequence control relay 39 causes the closing of the contact members 46 thereby closing one portion of the circuit including the actuating coil 41 of the anode energizing relay 48 adapted to control the energization of the anode 201 of the electric discharge device I15 explained more in detail hereinafter.

The operation of the sequence controlled relay 39 also causes the closing of the contact members 49 thereof thereby establishing a closed circuit for a portion of the circuit including the actuating coil 53 of the cathode energizing contactor 5|.

One additional circuit is established by the operation of the contactor 1, which circuit may be traced from the energized conductor 4 through contact member 33, actuating coil 52 of the time limit device I2 to the energized conductor 9. The time limit device I2 is provided with a dash pot 53 which delays the closing of the contact members 54 of the time limit device I2 for an interval of time sufficient in length to permit the motor I1 to attain substantially its balanced speed, namely a speed somewhere in the neighborhood of 95% to 98% of synchronous speed. The time limit device need not be of the type shown but may be any kind of a switch having a delayed action such as an electric clock, a small synchronous motor, an inductive time limit relay or an ordinary clock set in operation by the operation of the contactor 1. However, the adjustment of whatever time limit device may be selected should be such that contact members 54 do not close prior to the time that the motor I1 has attained substantially its balanced speed.

When the motor has attained its balanced speed, contact members 54 are closed thereby establishing an energized circuit from the conductor 3 through conductor 4, contact members 33 and 54, conductor 55, back contact members 56 of the sequence control relay 36, conductor 51, actuating coil 58 of the field contactor 30 and conductor 59 to the energized conductor I0. It is, therefore, clear that the field contactor 30 is immediately energized after the closing of the contact members 54 and remains energized through contact members 00. In other words, insofar as the foregoing discussion related to the starting of motor I1 only, the system disclosed is a time limit control starting scheme and in and of itself and independent of the other features of the system of my control does not constitute part of my invention but represents a system of control well known in the art.

Energization of the actuating coil 50 causes the operation of the field contactor 30 which contactor is so designed that contact members 52 will close before the contact members 29 open and upon deenergization contact members 29 will close before contact members 62 open. The closing of contact members 62 connects the field winding of motor 1 to the source of direct current by a circuit which may be traced from the positive conductor 0| through contact members 02, field rheostat 63, conductor 32, one of the slip rings of the field winding, the field winding, the other slip ring, and thence to the negative conductor 64. The field winding being itself energized with direct current will cause the motor I1 to pull into synchronism and the motor will operate its load in the conventional manner.

After the motor I1 has been synchronized and is operating its load in the desired manner and it is necessary, or found desirable, to also connect the motor I10 to the same source of alternating current and the attendant wishes to so synchronize motor I10 that it shall have its shaft in a definite relation with reference to the shaft of motor I1, the attendant actuates the starting switch or push button I05, thereby establishing an energizing circuit from the conductor 3 through conductor I04, starting switch I05, actuating coil I06 of the contactor I01, stop switch I00 and conductor I09 to the conductor 40 and thence to the energized conductor2. Contactor I01 will close contact members I thereby estab lishing a holding circuit for the actuating coil I05 and will also close the contact'members I30, I40 and I50 thereby energizing the armatures or stator winding I of the motor I10.

The secondary or rotor carrying the field winding will thus begin to operate-operating, as an induction motorto drive the shaft I90 to which the load may be coupled and also drive the bars or members I20 and I2I of magnetizable material of the Impulse generator I 23. The members I20 and I2I may be adjustably securedto the shaft I90 by adjusting means I24 so thrt the impulses generated in the windings I21 and I20 of the stator I25 of the impulse generator I23 will have a definite positional relation to the shaft of the motor and thus the rotor I00. The stator I25 is rigidly secured to the stator of the motor I10 by means of the connection I20.

An alternating current will be induced in the field wlnding of the rotor in the well known manner and the current induced will circulate in the discharge circuit including the contact members I29, resistor I3I and conductor I32.

The operation of the contactor I01 also closes the contact members I33 thereby establishing an energizing circuit for the actuating coil I52 of the time limit device 3I2 having the dash pot I53. The time limit device 3I2 may be similar to the time limit device I2 and has the same function as time limit device I2.

The closure of the contact members I33 also establishes an energizing circuit from the energized conductor I04 through contact members I33, conductor I10, actuating coil 50 of the oathode energizing switch 5| and contact members 49 to the energized conductor 40. The switch 5| will thus be caused to operate, closing contact members I13, IOI and I04 and opening the contact members I02 and I03. The closure of the contact members I13 establishes an energizing circuit from the energized conductor I04 through conductor "I, the primary of the transformer I12 and contact members I13 to condutcor 49. The cathode I14 of the electric discharge device I15 will thus be energized immediately after the closing of the lower contact members on the contactor I01. The cathode I14 is connected to the negative conductor 04 oi the source of direct current through conductor I11 and a portion )1 the resistor or potentiometer I18 and conductor I19. The adjustment of the conductor I19 determines the negative potential of the cathode I14.

The closing of the contact members IOI establishes a potential across the resistor or potentiometer I 10 by a circuit that may be traced from the positive conductor 0| through contact members IOI, resistor I10 and conductor I11 to the negative conductor 04. The opening of contact members I02 and I03 prevents energization of the portion of the system of control, shown in Fig. 1, that includes the electric discharge device 215.

The bias of the grid I05 with reference to the cathode I14 is determined by the potential across conductors I19 and I00 which is, in turn, determined by the potential existing between conductors I11 and I05. .The potential of conductor I00 is, however, a function of the operation of the impulse generators 23 and I23. Coil 20 of the impulse generator is connected across the resistor I01. The circuit for this connection may be traced from the conductor I11 through resistor I01, conductor I00, back contact members I09 of the cathode energizing controlled switch 25I conductor I9 I, coil 20 and negative conductor 04 to the conductor I11.

Coil I20 of the impulse generator I23 on the other hand is connected across the resistor 201, the circuit for this connection may be traced from conductor I02, conductor I00. coil I20. conductor I00, back contact members I of switch 25l, conductor I00 and resistor 201 to the conductor I02. Since the circuit between the grid I05 and the cathode I includes the conductor I10, a portion of the resistor I10, a portion of resistor I01, a portion of the resistor 201, and conductor I00, it is apparent that the bias of the grid with reference to the cathode is determined or influenced by the impulses generated in the coils 20 and I20. Therefore, if the members I20 and I2I have a difi'erent position on the shaft I00 with reference to the stator I25, than the position occupied by the members 20 and 2| on the shaft I0 with reference to the stator 25, it is apparent that when the rotors of the two machines are in exactly the same position, the impulses from the coils 20 and I20 will not be in synchronism or will not occur at the same instant. Simultaneous occurrences of the impulses of the two coils will only take place if the rotor of one of the motors is either behind or ahead of the position of the rotor of the other motor.

In the particular showing made in Figs. 1 and 2, the members 20 and 2|, and I20 and I2I of both the impulse generators appear to be in the same position on the respective shafts, but it should be noted that the field connections at the slip rings are different for the twomotors I1 and I10. It is, therefore, apparent that with machines of the same design and the same capacment ofthe adjustable conductors shown in cona circuit from the positive conductor 0| through nection with the resistors I10, I81 and 201 may be so made that the electronic discharge device I breaks down or becomes conducting only when the impulses from the coils 28 and I occur at the same instant, namely when the voltages of these impulse generators are additive does the discharge device I15 break down.

To prevent premature operation of the electric discharge device, energization of the saturation coils 21 and I21 of the impulse generators 23 and I23, respectively, as well as positive energization of the anode 201 is delayed for an interval of time of sufllcient length that the motor I10 may attain its balanced speed, namely a speed of'95% to possibly 98% of synchronous speed. This delay is accomplished by the time delay device 3I2.

After the necessary time delay, contact members I50 are closed by the time limit device 3I2 and in consequence actuating coil 01 of the anode energizing control relay 08 is energized. The circuit for this coil may be traced from conductor I00 through contact members I50, actuating coil '01 and contact members 06 to the energized conductor 00. Operation of the relay 00 establishes adjustable resistor 200, contact members 20I, contact members I80, conductor 202, coil I21 of the impulse generator I23, conductor 200 and coil 21 of the impulse generator 20 to the negative conductor 00. The impulse generators 20 and I20 and the stators 25 and I25 thereof are thus suitably energized with direct current to saturate the cores of the stator in any desired manner to aid in the production of a sharper impulse by the impulse coils 20 and I20.

Operation of the relay 00 also closes the contact members 200, thereby connecting the anode 201 to the positive conductor 0| by a circuit'that may be traced through the resistor 200, contact members 200 and actuating coil 200 of the high speed field switch 200 to the anode 201. The actuating coil 200 will, however, not be energized to operate the high speed field switch 200 but is energized only when the discharge device I10 becomes conducting by reason of the concurrent action of theimpulse coils 20 and I20.

For the assumptions made, it is apparent that at this stage of operation, motor I1 operates as a synchronous motor whereas motor I10 operates as an asynchronous motor operating somewhere near synchronous speed. The first instant after the operation of the time limit device 0I2 that the rotor shaft of the two motors I1 and I10 hold the position desired, the impulses of the coils 20 and I20 will occur at the same time thereby changing the "bias of the grid I00 by an amount sufilcient to cause the breakdown or the discharging of electric discharge device I15. When such discharge occurs, actuating coil 200 is energized and in consequence, contact members 200 are closed thereby establishing an energizing circuit for the field winding of the motor I10 by a circuit that may be traced from the positive conductor 6| through contact members 200, field rheostat I68, conductor I02, the field winding and thence to the negative conductor 80. The high speed field switch 208 is not designed to continuously carry the field current and is, therefore, provided with contact members 2I0 for energizing the main field switch 05. The energizing circuit may be traced from conductor I00 through contact members I50 and 2I0 and the actuating coil 00 of the main field switch 05 to the energized conductor 00. The main fieldswitch 05 will thus operate to close the contact members I52 thereby establishing a low resistance shunt for the contact members 200 of the high speed field switch 200. Furthermore, operation of the main field contactor or switch 05 opens the discharge circuit for the field winding at the contact members I29. A holding circuit for coil 00 is also established through contact members 2I I.

If the attendant wishes to start motor I10 first and thereafter wishes to synchronize motor I1 in such a manner that the motor shaft may have a definite relation to the rotor shaft oi motor I10, he actuates the starting switch I05 thereby synchronizing motor I10 by a time limit control which is effected by the time limit device 3I2 in exactly the manner hereinbefore discussed in connection with the starting of motor I1. However, in this instance the closure of the contact members I33 establishes a circuit from the energized conductor I00 through contact members I33, back contact members ll of the sequence control relay 38 which will of course, be deenergized and actuating coil 02 to the energized conductor I0. The sequence control of relay 36 is, therefore, caused to operate when motor I10 is started first with the result that time limit starting control of motor I1 is prevented by the opening of contact members 50 and the closure of contact members 2l2 and 2I5 will set up the necessary circuits for properly' synchronizing motor I1 in a definite relation to the motor I10 when itisdesiredtostartmotor I1.

'ergized by acircuit that may be traced from the After the'motor I18 has become properly synchronized, the attendant may actuate the starting switch 8 and silent the starting of the motor I1 as an induction motor exactly as herein before explained. However, since contact members 2I5 are closed when motor I18 is already operating, a circuit is established from theenergized conductor 4 through contact members 33, conductor 2I1, actuating coil 2I8 and contact members 2I5 to the energized conductor I8.. The cathode energizing control relay 25I is thus caused to operate, closing the contact members 2I8, 224 and 248 and opening the contact members I89 and I95.

The opening of contact members I88 and I88 disconnects certain portions of the control associated with the discharge device I15. Closure of contact members 248 merely provides a closed circuit in the portion of this circuit including the coils I21 and 21 of the impulse generators I23 and 23, respectively.

The closure of contact members 2l8 establishes an energizing circuit from the conductor 4 through conductor 22I, primary oi transformer 2I8, contact members 2I8 to conductor I8, thereby causing the energization of the cathode 228 of the electric discharge device 215 from the transformer 2I8. A negative potential 01' a certain value is applied to the cathode 228 by a circuit that may be traced from the negative conductor 84 of the source of direct current through conductor 228, a portion of the resistor 222 and conductor223 to the cathode 228. The closure of contact members 224 merely connects the potentiometer or resistor 222 across the source of direct current, namely across conductors 8| and 84 by a circuit including contact members 224 and the potentiometer-resistor 222 and conductor 228.

The bias of the grid 33 with reference to the cathode 228 is determined by the potential across conductors 223 and 225 which, in turn, is determined by the potential across the conductors 228 and 225. The potential of conductor 225 with reference to the conductor 228 is, however, determined by the variations in potential caused by the coils 28 and I28 of the impulse generators 23 and I23, respectively. Coil 28 is connected across the resistor 221. The circuit for this connection may be traced from conductor 228 through conductor 84, coil 28, conductor I 3|, conductor 228, contact members I82, conductor 228 and resistor 221 to the conductor 228. Coil I28 is connected across the resistor 238. The circuit for this connection may be traced from conductor I82 through conductor I83, coil I28, conductor I84, contact members I3,conductor 23I and resistor 238 to the conductor I32. By properly adiusting the adjustable conductors on the respective resistors 222, 221 and 238, the potential of the grid 233 may, of course, be so selected that discharge device becomes conducting or discharges only when the impulses of the coils 28 and I28 occur at the same time. By properly adjusting the relative position on the shaft of the motor of the rotating members, of the impulse generators, it is apparent that motor I1 may be pulled into synchronism at any selected position with reference to the motor I18. By any position in this instance, it is, of course, meant any position permitted by the design,.namely the number of poles of the two machines.

When the impulses of the impulse generators occur at the same time, discharge device 215 becomes conducting and in consequence actuating coil 238 of the high speed field switch 231 is enpositive conductor 8| through resistor 234, contact members 235, actuating coil 238, anode 238, cathode 228, conductor-223. and a portion of the resistor 222 to the negative conductor 84. Of course, contact members 235 would only be closed ii the time limit device I2 has operated to establish an energizing circuit for the actuating coil 2I3 of anode energizing control relay 2I4. Fur-, thermore, the operation of relay 2I4 closes a circuit from the positive conductor 8| through adjustable rheostat 338, contact members 238 and 248, conductors 24I and 282, coil I 21 of the impulse generator I23, conductor 283 and coil-21 oi the impulse generator 23 to the negative conductor 84.

' The instant high speed field switch 231 operates, contact members 34I are closed thereby establishing an energizing circuit for the main field switch 38 by a circuit that may be traced from conductor 4 through contact members 33, 54 and 3, actuating coil 53 of the main field switch 38 and conductor 53 to the energized conductor I8. Operation of the main field switch 38 establishes a shunt for the contact members 242 of the high speed field switch 231 thereby relieving these contact members of the duty of continuously carrying a comparatively heavy current. Further, operation of the main field switch 38 opens the discharge circuit in the manner hereinbefore explained.

The operation 01' the main field switch 38 of course energizes the field winding of the motor l1 and causes this motor to pull into synchronism in such a manner that the shaft of motor I1 has a definite" selected relation with reference to the shaft I8.

I am, of course, aware that others skilled in the art, particularly after having had the benefit of the teachings of my invention may devise other circuit diagrams than the particular diagram I have shown for accomplishing the novel results of my invention. However, I do not wish to be limited to the particular circuit diagrams shown or described but wish to be limited only 'by the appended claims and the pertinent prior art.

I claim as my invention:

1. In a system of control for a pair of synchronous motors, in combination, a synchronous motor having an armature winding and a field winding, a source of alternating current, means for connecting the armature to the source of alternating current, a source of direct current, means for connecting the field winding to the source of direct current, a second synchronous motor having an armature winding and a field winding, means for connecting the armature winding of the second motor to the source of alternating current, a generator coupled to each of said motors and therefore driven in synchronism with said motors, said generators having voltage characteristics indicative'of the position of certain points on the motor shafts with reference to corresponding planes including the axes of the respective motors, an electric discharge device responsive to the voltage of the said generators and thus adapted to become energized when a certain relation exists between the motor shafts, and switching means responsive to said discharge device adapted to connect the field winding of the second motor to said source of direct current.

2. In a system of control for a pair of synchronous motors, in combination, a synchronous motor having an armature winding and a field source of direct current, a second synchronous motor having an armature winding and a field winding, means for connecting the armature winding of the second motor to the source of alternating current, a generator coupled to each of said motors and therefore driven in synchronism with said motors, said generators having voltage characteristics indicative of the position of certain points on the motor shafts with reference to corresponding planes including the axes of the respective motors, an electric discharge device responsive to the voltage of the said generators and thus adapted to become energized when a certain relation exists between the motor shafts, and switching means responsive to said discharge device adapted to connect the field winding of the second motor to said source of direct current, means responsive to a certain combined voltage of said generators adapted to energize the field winding of the second motor from said source of direct current whereby said syn- "chronous motors, in combination, a synchronous motor having an armature winding and a field winding, a source of alternating current, means for connecting said armature winding to said source of alternating current, a source of direct current, means for connecting said field winding to said source of direct current, an impulse generator coupled to the rotating element of said synchronous motor, a second synchronous motor having an armature winding and a field winding, means for connecting the armature winding of said second synchronous motor to said source of alternating current, switching means for connecting the fleld winding of said second synchronous motor to said source of direct current, a second impulse generator so coupled to the second synchronous motor that the impulse generated thereby will be in electrical phase with the impulse generated by the first impulse generator when corresponding points on the rotating elements of the synchronous motors are out of mechanical phase, electronic means responsive to the combined magnitude of the in-electricalphase impulses of the impulse generators when one of said generators is operated at the synchronous speed of the synchronous motor to which it is coupled and the other is operated at a speed near the synchronous speed of the synchronous motor to which it is coupled, said switching means being interconnected with said electronic means and adapted to be operated in response to energization of said electronic means.

CHARLES c. snro'r'r. 

